This invention relates to a process for forming fine thick-film conductor patterns. More particularly, the invention relates to a process for forming fine thick-film conductor patterns on ceramic circuit substrates or ceramic green sheets for use in mounting IC (e.g., LSI) chips and other electronic components or interconnecting them. The invention also relates to a process for forming connecting bumps on the surface of ceramic substrates for flip-chip bonding.
With a recent trend in the electronic industry towards fabricating smaller devices of higher performance, the practice of using semiconductor packages having more pins with smaller pitches or accommodating multiple chips on a single substrate has accelerated. The LSI bonding technology is also making a shift from the conventional wire bonding process to wireless bonding such as the TAB (tape-automated bonding) or flip-chip bonding process which is suitable for multi-chip devices and high-density packaging. As the packing density of electronic devices thus increases, it is required in the art of ceramic circuit substrates used to mount them to develop a technology capable of forming fine thick-film conductor patterns such as fine interconnecting lines with a line width of no more 100 .mu.m and bumps or electrodes with a diameter of no more than 100 .mu.m.
The formation of conductor patterns for circuit on ceramic substrates is generally performed by a thin-film deposition, plating, or thick-film deposition process.
In the thin-film deposition process, a metallized layer is formed to a thickness on the order of several microns on the ceramic substrate by vacuum deposition, sputtering, ion plating, or a similar vapor-phase technique, and it is then patterned by a photolithographic technique using a photoresist. This process is capable of forming highly precise conductor patterns but, on the other hand, it has the following disadvantages: the conductor patterns have low adhesion to the substrate; the method involves an increased number of steps and requires the use of an expensive thin-film forming apparatus and, hence, the fabrication cost is high; and the formed conductor patterns are so thin that it is difficult to pass a large current therethrough.
In the plating process, a thin metallized layer is formed on the ceramic substrate from a solution by means of an electrochemical technique and is then patterned in the same manner as described above. This process also suffers from most of the above-described disadvantages of the thin-film deposition process.
In the thick-film deposition process, a conductive paste comprising a conductor component (metal powder) dispersed in a liquid vehicle of an organic solvent containing an organic resin as a binder is screen-printed onto the ceramic substrate followed by firing to form a relatively thick conductor pattern on the substrate.
The ceramic substrate to be used in the thick-film process is not necessarily a sintered ceramic substrate. Instead, a ceramic green sheet may be used to apply a conductive paste to form a paste pattern thereon and it is then co-fired with the applied conductive paste. This technique is employed in a well-known green sheet multilayer lamination process, in which a plurality of ceramic green sheets each having a conductive paste pattern formed by screen printing are laminated and co-fired to produce a multilayer ceramic circuit substrate.
The thick-film process allows for low-cost formation of a conductor pattern having an adequate strength of adhesion to the ceramic substrate and capable of passing a large current therethrough.
When a ceramic green sheet is used for screen printing with a conductive paste in this process, the liquid component of the printed conductive paste is quickly absorbed by the green sheet, thereby making it possible to form a conductor pattern with a small line width and spacing of no more than 150 .mu.m. In contrast, when a conductive paste is applied to a sintered ceramic substrate, the liquid component will not be readily absorbed by the substrate but spreads laterally to cause "blur" or "slump" of the paste, thereby making it difficult to form the as-designed conductor pattern having a line width and spacing of no more than 150 .mu.m.
Under the circumstances, an attempt has been made to combine the formation of conductor patterns by the thick-film process with the photolithographic technology which has predominantly been employed in the thin-film process. According to this method, a photoresist layer is first formed on the surface of a sintered ceramic substrate, and a photolithographic technique including imagewise exposure to radiation (light) and development is then applied to the photoresist layer so as to form grooves therein having a pattern which corresponds to the desired fine conductor pattern. Subsequently, the grooves are filled with a conductive paste by squeezing the paste into the grooves with a squeegee, and the substrate is fired to sinter the conductor component in the paste and simultaneously remove the photoresist layer and any other organic substances by decomposition (see, for example, Japanese Patent Applications Laid-Open (Kokai) Nos. 223391/1992, 223392/1992, and 223393/1992).
The firing to pyrolytically remove the photoresist layer is generally performed at around 900.degree. C. in an oxidizing atmosphere. However, it takes a considerably long time for the photoresist layer to be completely burnt out. If the conductor component of the conductive paste is susceptible to oxidation as is the case with Cu or Mo-Mn, it will be readily oxidized during the firing in an oxidizing atmosphere, thereby resulting in the formation of a conductor pattern having a substantially increased resistivity. To avoid this problem, a relatively expensive conductive paste in which a noble metal such as Au, Ag, Ag/Pd, Pd, or Pt is used as the conductor component must be used in this method.
It has also been proposed with respect to this method that the firing step to burn out the photoresist layer in an oxidizing atmosphere is followed by a step of reducing the oxidized conductor metal, but this approach is less preferred since it increases the number of process steps.
Japanese Patent Application Laid-Open (Kokai) No. 240996/1990 proposes the use of a positive-working photoresist in the above-described method of forming thick-film conductor patterns by the photolithographic technology. According to this method, after patterned grooves are formed in the photoresist layer (which is positive-working) by imagewise exposure and development and they are filled with a conductive paste by squeezing, the photoresist layer is subjected to blanket exposure, whereupon it becomes soluble in a developing solution and hence can be removed by a wet process rather than firing. After removal of the photoresist layer by a wet process, firing is performed in order to sinter the conductor component of the conductive paste and, in this case, the firing atmosphere may be non-oxidizing. As a result, conductive pastes in which the conductor component is an oxidation-susceptible metal such as Cu or Mo-Mn can be employed to form thick-film conductor patterns without oxidation of the conductor metal which leads to an increased resistivity.
However, this method still has the following problems.
(1) In the step of squeezing the conductive paste into the patterned grooves of the photoresist layer, a skin of the paste will also be unavoidably deposited on the top surfaces of the photoresist layer. Unless this skin of the conductive paste is removed prior to firing, it remains after firing to form a thin conductor layer between interconnecting lines in the designed thick-film conductor pattern, which may cause short circuit.
The skin of the conductive paste deposited on the photoresist layer can be removed by polishing the surface of the photoresist layer, but this increases the number of process steps. In addition, it is difficult to achieve uniform polishing and, therefore, surface irregularities tend to occur in the formed conductor pattern.
(2) The solvents generally used in the conventional conductive pastes are polar solvents, particularly alcoholic or ester solvents such as terpineol, butylcarbitol, and dibutyl phthalate, since they have a high dissolving power for the binder resin in the paste. The high dissolving power of these solvents, however, causes the photoresist layer having grooves filled with a conductive paste to be dissolved into the solvent of the conductive paste, thereby distorting the conductor pattern and impairing the sharpness of the profile thereof.
Furthermore, if a skin of the conductive paste is undesirably deposited on the photoresist layer as mentioned in (1), the solvent of the conductive paste present as the skin will dissolve the underlying photoresist layer, whereupon the deposited conductive paste will firmly adhere to the photoresist layer. This makes it difficult to ensure that the photoresist layer is completely removed with a developing solution in a subsequent step. In addition, the problem described in (1) is aggravated.
(3) The squeegee used to fill the grooves formed in the photoresist layer with the conductive paste is usually a rubber plate. Such a squeegee can successfully fill grooves having a width or diameter of no more than 100 .mu.m. However, if the width or diameter of the grooves exceeds 100 .mu.m, it is difficult to completely fill the grooves with the conductive paste using this squeegee. This is because the rubber squeegee will be flexed and partly get into the wide grooves so that part of the already filled conductive paste is scraped out of the grooves by the squeegee.
In common ceramic circuit substrates, interconnecting patterns having a width or diameter of no more than 100 .mu.m exist together with those having a greater width or diameter. Therefore, the use of a rubber squeegee makes it difficult to completely fill all the grooves of the photoresist layer with the conductive paste.
It is conceivable to use a metallic plate as the squeegee in place of a rubber plate. Using the metallic plate, one can fill the grooves with the conductive paste almost completely even if the width or diameter of the grooves varies widely. However, the use of such a hard metallic squeegee makes a gap from the top surface of the photoresist layer when squeezing, resulting in the formation of a thick skin of the conductive paste deposited on the photoresist layer. As a result, the problem mentioned in (1) is aggravated, thereby increasing the possibility of short circuit. If the metallic squeegee is strongly pressed down during squeezing so as to avoid the formation of the thick skin of the conductive paste, the photoresist layer is damaged or deformed and the resulting interconnecting pattern does not have the desired accurate shape.
(4) The photoresist layer being formed is so thick that it is difficult even by the photolithographic technology to form a fine conductor pattern having a line spacing of 25 .mu.m or less.
(5) A photomask is necessary to perform photolithography (in the imagewise exposure step), and a different photomask has to be fabricated for each different conductor pattern to be formed on the substrate. However, this is not economically advantageous in the production of multiple varieties in small lots.
(6) In order to achieve a satisfactory resolution, the photomask has to be placed in intimate contact with the photoresist layer formed on the ceramic substrate. However, due to the inherent subtle distortion of the ceramic substrate, there may be a gap between the photoresist layer and the photomask to deteriorate the resolution.
Japanese Patent Application Laid-Open (Kokai) No. 283946/1992 teaches a method of forming a fine thick-film conductor pattern on a ceramic green sheet rather than on a sintered ceramic substrate by the above-described photolithographic technology using a positive-working photoresist. According to this method, the conductive paste is squeezed into the grooves of the photoresist layer, which is subsequently removed, after blanket exposure, by a wet process using a developing solution while leaving a predetermined pattern of the conductive paste on the ceramic green sheet. A plurality of such ceramic green sheets are laminated by thermo-compression bonding, and the resulting laminate is fired to produce a multilayer interconnected ceramic substrate.
However, even this method is not perfect and has the following problems.
(7) The application of the photolithographic technology to a ceramic green sheet may cause the photoresist layer to be dissolved in the green sheet or cause the binder in the green sheet to be dissolved in the developing solution being used to remove the photoresist. In either case, the conductor pattern being formed on the green sheet tends to be deformed.
(8) The aforementioned problem of short circuit also occurs on account of the excessive conductive paste undesirably deposited on the photoresist layer.
(9) In the laminate, the thick-film conductor patterns formed on the individual ceramic green sheets are so raised that the individual green sheets will become distorted, thereby increasing the chance for delamination, warpage, and other defects to occur during the subsequent firing step.
As already described, the requirement of the recent model of semiconductor devices for smaller size and higher packing density has motivated the increasing shift from wire bonding or surface mounting technology (SMT) toward flip-chip bonding which is capable of higher-density packaging of IC chips.
FIG. 5 shows an IC chip that is joined to a ceramic circuit substrate by the flip-chip bonding scheme. A ceramic circuit substrate 31 has bumps 32 formed on one surface for connection to an IC chip, and an IC chip 33 also has bumps 34 formed on one surface. The two groups of bumps are electrically connected and fixed in position through a solder 35. The substrate 31 may be a glass substrate.
A common method of connecting the bumps 34 on the IC chip 33 to the bumps 32 on the substrate 31 comprises depositing the solder 35 usually on the bumps 34 of the IC chip 33, then placing the IC chip 33 on the substrate 31 such that the bumps 34 of the IC chip 33 having the solder 35 thereon are in contact with the bumps 32 on the substrate 31, and heating to fuse the solder 35. Since a space is ensured between the substrate 31 and the IC chip 33, flux residue and solder scum which may remain after soldering can be completely removed by cleaning to assure improved reliability. The flux is normally used with a solder.
For connecting a ceramic circuit substrate to a mother board, the DIP (dual-in-line package) and PGA (pin-grid array) techniques have heretofore been employed. However, for this connection, too, the need to satisfy the requirement for smaller size, higher packaging density, and lighter weight has prompted an attempt to perform the flip-chip bonding scheme, which is capable of direct surface mounting of the substrates on the mother board. This scheme is also called a BGA (ball-grid array) technique.
FIG. 6 illustrates how a ceramic circuit substrate is joined to a mother board by the flip-chip (BGA) technique. A ceramic circuit substrate 31 has bumps 32 formed on one surface for connection to a mother board 41, and the mother board 41 also has bumps 42 formed on one surface. The two groups of bumps are electrically connected and fixed in position through a solder 35. The bumps 32 on the substrate 31 can be connected to the bumps 42 on the mother board 41 by the same method as described with reference to FIG. 5.
FIG. 7 schematically shows an example of the ceramic circuit substrate 31 to be used for connecting to an IC chip by the flow-chip technique. In order to achieve high-density packaging, plural layers of internal interconnecting lines 37 are formed within the substrate 31, and individual internal interconnecting lines 37 are connected through via holes 38 filled with a conductive material. In order to connect the interconnecting lines 37 to an electronic device (not shown) by the flip-chip scheme, a bump 32 is formed at the end of each via hole 38 which is exposed on the surface of the substrate. As can be seen from FIG. 7, the bumps 32 to be formed on a substrate for high-density packaging are small in size and positioned with a narrow spacing. It is therefore necessary to form the bumps in high precision and with a high density.
The term "bumps of high precision" refers to those pads which have little fluctuation with respect to diameter, thickness (height), pitch (spacing), and other parameters, and the term "bumps of high density" refers to those pads which are small in diameter and pitch.
In contrast, the bumps used to mount the ceramic circuit substrate 31 on the mother board 31 by the flip-chip (BGA) technique as shown in FIG. 6 generally have a larger diameter and a lower density, yet they are also required to be formed with reasonably high precision.
Conventionally, the formation of bumps on a ceramic circuit substrate has been performed by screen printing with a conductive paste. The paste is then fired to form the bumps on the substrate, and the bumps are optionally overcoated, e.g., by Ni and Au plating or Cr-Cu vapor deposition. However, this screen-printing technology involves the following difficulties.
(10) FIG. 8 shows the connection of bumps 32 formed on a ceramic circuit substrate 31 by screen printing (using a screen mask having openings arranged in a predetermined pattern) to bumps 42 formed on a mother board 41 through a solder 35, which is deposited on the bumps 42 on the mother board 41. The print thickness attained by screen printing tends to fluctuate because the amount of the conductive paste that is extruded through the openings of the screen mask differs from one opening to another. As a result, the height of bumps 32 formed on the surface of the substrate 31 is not uniform but low bumps 32a intermingle with high bumps 32b and 32c. If the ceramic circuit substrate 31 having these bumps is mounted on the mother board 41 as shown in FIG. 8, a gap 36 forms between the low bumps 32a on the substrate 31 and the solder 35 deposited on the bumps 42 on the mother board 41, and this leads to connection failure. In addition, the shape of bumps is likely to be irregular.
(11) In the screen printing operation, the squeegee is moved as it holds down the screen. However, the screen mask flexes under the depressing force of the squeegee and, hence, the pattern being printed is prone to be misplaced relative to the preset position, thereby deteriorating the positional precision of the bumps.
(12) As already mentioned, when the conductive paste is printed onto a ceramic circuit substrate 31 which has been sintered, the solvent in the conductive paste will spread laterally on the substrate surface to cause "blur" or "slump", and bumps 32 that are finally formed may cause short circuit if their spacing is 100 .mu.m or less.
Some of these problems can be solved by forming bumps by vapor deposition on a ceramic circuit substrate. However, vapor deposition is not practically feasible for its high cost and a prolonged time required to give the necessary bump thickness.